High operating temperatures can severely affect the performance, power consumption and reliability of a circuit system. With the continued scaling of integrated circuit technologies, high power density and the resulting difficulties in managing temperatures have become a major challenge for designers at all design levels. Historically, temperature sensors such as thermal couples have been used to measure the thermal resistance of electronic packages for thermal characterization. The measurement result is, however, prone to errors and not sufficiently reproducible. One problem is a potential temperature distribution at the package case while the thermocouple measures the temperature just at its contact point to the case. Another problem is that thermocouple bead is often not sufficiently insulated against the cold plate and could therefore be cooled from the wire and cold plate side. The interface thermal resistance between the case and the thermocouple may also play a role. A further issue with thermocouples is that they cannot measure the temperature of the internal structure, yet by design the dominant heat flow path is from the junction, through many materials and material interfaces before passing into a PCB or heat sink. Thermocouples are therefore a “blunt instrument” when it comes to thermal design verification.
The thermal transient measurement technique has been introduced to overcome the disadvantages of the direct temperature measurement. In a thermal transient measurement, a step power is applied to a structure of interest and the response of the structure is recorded and analyzed. For example, an electronic package containing a bipolar junction transistor is powered to a certain power value and left until it reaches a steady state condition. The package is then powered off and the resulting junction temperature response is measured using a specialized tool such as the commercial T3Ster® system available from Mentor Graphics Corporation of Wilsonville, Oreg. The thermal transient measurement can generates a curve of normalized transient thermal impedance (Zth) with the transient thermal impedance being calculated from the temperature change in time.
The Zth curve is in time domain and does not show structural information. It has been demonstrated that a thermal system can be treated as a distributed thermal RC (resistance-capacitance) network. Thermal resistance and thermal capacitance of thermal model elements on the heat flow path determine step power response of the system. The Zth curve can be converted to a structure function (also referred to as cumulative structure function or thermal structure function) showing cumulative thermal capacitance as a function of the cumulative thermal resistance. The structure function can be divided into parts or portions corresponding to layers of the thermal model elements on the heat flow path. This allows the identification of partial thermal resistances and partial thermal capacitances on the heat flow path not only inside the device package like die attach, but also outside electronic components such as PCB board, surface-air boundary layer, and contact thermal resistance. It can also help with calibrating the thermal model for thermal transient response simulations.
It is not trivial to determine which part of the structure function corresponds to which thermal model element, however. Several conventional approaches all have their own advantages and disadvantages.
The first one is based on isothermal surfaces. Knowing the resistance value that a structure function feature occurs at, one can determine the temperature value at which this resistance relates to (Tj-(Resistance/Power)). Those objects that are bisected by the simulated iso-surface of that temperature, and carry the majority of the heat flow in the steady state power on condition, are those that may be responsible for that resistance. This would be a reliable approach if the temperature variation at the object interface into which heat flows had a uniform temperature. In many parts of a package model this is not the case and so such an approach does not always indicate correctly which objects correlate to which structure function resistances.
An alternative approach is to note that time at which the resistance in question “occurs at” then to study the simulated heat flux distribution at that time point. The fore front of the heat flux field (using the “power on” approach to determine the thermal impedance curve) should be experiencing the object that is responsible for that resistance, at that time. If the resistances that are apparent on the structure function are sensed at one point in time then again, this would be a reliable approach. However the structure function resistances are built up as heat starts to pass through the object, soaks into it then passes through it.
A more full-proof approach is to make perturbations to the numerical model at known locations and compare how these relate to differences in resulting structure functions. More reliable as is, the perturbation method is a computationally expensive approach.